Method and apparatus for synchronizing droplet formation in a liquid stream

ABSTRACT

A source of energy is selectively applied to a liquid stream to reduce the surface tension of the liquid and is applied before the stream would randomly break up into droplets. Both the quantity of energy applied and the time period that the energy is applied are controlled to control the time to droplet breakoff and the time between droplets by selectively reducing the surface tension of segments of the stream. The source of energy can be high intensity light, which is converted by the stream to heat energy, or a source of heat (resistive or inductive) with the resistive heat being applied to the stream by conduction and the inductive heat being converted by the stream to heat energy.

United States Patent [1 1 Eaton [451 Apr. 15, 1975 METHOD AND APPARATUSFOR SYNCHRONIZING DROPLET FORMATION IN A LIQUID STREAM James H. Eaton,Armonk, NY.

International Business Machines Corporation, Armonk, NY.

Jan. 31, 1974 Inventor:

Assignee:

Filed:

Appl. No.:

US. Cl. 346/1; 239/13; 239/133; 346/140 Int. Cl. G01d 15/18 Field ofSearch 346/75, 140, 1; 239/133, 239/134, 135, 13; 118/620, 641, 302

References Cited UNITED STATES PATENTS I Ii INK 3.731.876 5/1973Showalter 239/13 Primary E.raminerJoscph W. Hartary Attorney, Agent, orFirm-Frank C. Leach, Jr.

[57] ABSTRACT A source of energy is selectively applied to a liquidstream to reduce the surface tension of the liquid and is applied beforethe stream would randomly break up into droplets. Both the quantity ofenergy applied and the time period that the energy is applied arecontrolled to control the time to droplet breakoff and the time betweendroplets by selectively reducing the surface tension of segments of thestream. The source of energy can be high intensity light, which isconverted by the stream to heat energy, or a source of heat (resistiveor inductive) with the resistive heat being applied to the stream byconduction and the inductive heat being converted by the stream to heatenergy.

19 Claims, 6 Drawing Figures HIGH INTENSITY UGHT SOURCE SUPPLY INTENSITYCONTROL DEFL MEANS I DEFL. 22 J SIGNAL SOURCE T0 INK RETURN PATENTEDAPRI 5&975

SHEEIlflF 2 man INTENSITY ucm SOURCE i6 1 FIG. 1

17 1a INK SUPPLY INTENSITY MOD. CONTROL 1 i5 ,0 DEFL. o o o MEANS DEFL.22 21 f SIGNAL SOURCE T0 INK l RETURN W HIGH INTENSITY INK LIGHT SOURCE32 SUPPLY 51 comm DEFL. SIGNAL SOURCE PS-JEKHEMFR] 5:975 3.878.518 saw 2i 2 40 CONTROL FlG.4

50 r CONTROL METHOD AND APPARATUS FOR SYNCHRONIZING DROPLET FORMATION INA LIQUID STREAM ln some ink jet printing embodiments. the droplets areselectively deflected in a direction perpendicular to the paper motionto place the droplets on the desired position of the paper. Wheredroplets are not wanted. they are selectively deflected into a gutter tocatch them before they can impinge on the paper.

In other embodiments. an array of nozzles is used to form a dropletstream for each desired spot position on the paper. Thus. ifall dropletswere to hit the paper. the paper would be uniformly covered with ink.Printing is accomplished by selectively removing unwanted droplets fromthe stream. usually by deflecting them into a gutter.

The most common means for selectively deflecting the droplets is toselectively place an electrical charge on the droplets and then to passthe droplets through a uniform electric field for deflection as shownand described in U.S. Pat. No. 3.596.275 to Sweet. The amount ofdeflection for a given droplet is then proportional to the previouslyselected electric charge and inversely proportional to its mass andvelocity. The charge on a droplet is determined by the electric field onthe droplet at its moment of breakoff from the stream.

The usual method of charging the droplet involves applying a voltage toa cylinder (charging tunnel) surrounding the point of breakoff of thedroplet from the stream. Thus. it is important to control both the timeand position of the breakoff so that a given timed voltage sequence willappropriately charge the droplets. If the droplets do not break off atthe correct time and position. they can receive an incorrect charge and.thus. be deflected to an undesired position.

The correct time and position is determined by synchronizing thedroplets so that they pass through the cylinder (charging tunnel) atuniform intervals of time and with the correct phase. in addition. thedisturbance causing the breakoff is modified in amplitude to give thedesired breakoff point within the charging tunnel.

Various means for obtaining synchronization of the droplets have beenpreviously suggested. For example. mechanical forces have been appliedto a nozzle by a piezoelectric device. for example. at a desiredfrequency.

When using a mechanical arrangement for creating the breakup ofthcstream into droplets. the mechanical force applied to one nozzle toproduce a vibrating frequency of the nozzle can have an effect on anadjacent nozzle of the ink jet streams if the ink streams are disposedon five mil centers. for example. The transmission of the mechanicalvibrations to an adjacent nozzle can alter the phase and breakoff pointof the droplets of an adjacent nozzle.

The present invention satisfactorily solves the foregoing problem byproducihg synchronous formation of the droplets without any mechanicalforce being applied. Thus. with the method and apparatus of the presentinvention. the nozzles for more than one ink stream. if such isrequired. can be placed very close to each other without the dropletforming means for one of the ink streams having any effect on any oftheadjacent ink streams. Thus. droplet formation from each ink stream. ifmore than one is required. can be effectively controlled with the methodand apparatus of the present invention.

The present invention accomplishes the foregoing through causing athermal change or disturbance within the ink stream prior to the timethat the stream would randomly break up into droplets. The randombreakup of a stream into droplets depends upon its surface tension. itsvelocity. and its diameter with the breakup occurring after the streamleaves a confined passage unless the passage should be coated with amaterial that the liquid does not wet such as Teflon. for example.

By regulating the thermal change or disturbance in the ink stream. thebreakup ofthe stream into droplets is controlled to cause synchronousformation of the droplets and break off at the desired point. Thethermal change or disturbance in the stream is controlled throughregulating a source ofenergy. which produces this thermal change ordisturbance. as to the time it is applied. the length of the segment ofthe stream to which it is applied. and the quantity of energy applied.

By creating the thermal change or disturbance in spaced segments of thestream. the temperature of the spaced segments of the stream isincreased so that the surface tension of the spaced segments of thestream is reduced to cause synchronous formation of the droplets. Sincethe surface tension of the stream decreases as the temperature of thestream increases. the source of energy. which creates the thermal changeor disturbance in the spaced segments of the stream. results in alowering of the surface tension of the spaced seg ments of the stream tocause breakup of the stream into droplets in the desired relation. Theperiod between successive applications of a quantity of energy and thestream velocity control the spacing between the droplets. The breakoffpoint is determined primarily by the energy applied in each pulse.

The surface tension of a stream is directly proportional to the internalpressure within the stream so that the decrease in the surface tensionin a segment of the stream causes a reduction in the internal pressurein that segment of the stream. This decrease in the internal pressurehas the same effect as an increase in the diameter ofthe segment of thestream since the internal pressure of the stream is inverselyproportional to the diameter of the stream. By reducing the internalpressure ofa first segment of the stream. a second and adjacent segmenthas an increase in its internal pressure relative to the first segmentto force the liquid in the second segment to the first segment of thestream. This results in a further reduction in the pressure in thealready lowered internal pressure of the first segment of the streamsince it increases the diameter of the first segment of the stream toaccommodate the larger volume so that a positive feedback is provided tocause the stream to break up into droplets.

By selecting the segments or sections in which the source of energycreates a thermal change or disturbance so that the segments or sectionsdefined by the selected application of the source of energy are normallyno greater than the average length of the droplets when they break uprandomly. synchronization of the formation of the droplets from thestream can be obtained. Thus. the small changes in surface tension inthe spaced segments of the stream trigger a positive feedback and leadto controlled droplet formation. The breakoff point is determinedprimarily by the amplitude of the thermal changes since an increase inamplitude causes the breakoff to occur closer to the nozzle.

The present invention contemplates using a modulated heat or highintensity light source as the source of energy. When a resistive heatsource is employed. the heat is supplied to the ink stream by conductionto cause the thermal change or disturbance in the stream. When the heatsource is inductive. heat conversion occurs within the stream to producethe thermal change or disturbance in the stream. When modulated lightfrom a high intensity light source is the source of energy to cause thethermal change or disturbance of the stream. heat conversion occurswithin the stream. Of course. the stream must not be transparent to thelight for there to be the conversion ofthe light to heat within thestream.

An object of this invention is to synchronously form droplets from aliquid stream by the intermittent application of a source of energydirectly to the stream.

Another object of this invention is to selectively alter the surfacetension of segments of a liquid stream to synchronously form dropletsfrom the stream.

A further object ofthis invention is to produce a thermal change in aliquid stream before random breakup of the liquid stream would occur.

The foregoing and other objects. features. and advan tages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention as illustrated inthe accompanying drawings.

In the drawings.

FIG. I is a schematic view showing a source of energy being applied to aliquid stream to cause synchronous formation of droplets therefrom andthe droplets producing printing on a recording surface.

FIG. 2 is a schematic view of another form of the source of energy beingapplied to the liquid stream to cause synchronous formation of dropletstherefrom.

FIG. 3 is a fragmentary sectional view of a portion of a nozzle andshowing a modification of the present invention in which an electricheater is the source of energy to a liquid stream and taken along line33 of FIG. 4.

FIG. 4 is a schematic end elevational view of the nozzle of FIG. 3 withthe electric heater and its control.

FIG. 5 is a fragmentary sectional view of another embodiment fordisposing an electric heater within a nozzle and taken along line 55 ofFIG. 6.

FIG. 6 is a schematic end elevational view of the noz zle of FIG. 5 withthe electric heater and its control.

Referring to the drawings and particularly FIG. I, there is shown an inksupply 10. Ink. which can be magnetic or non-magnetic. is supplied underpressure from the ink supply to a nozzle II.

A pressurized ink stream I2 passes from the nozzle ll through an o ening[4 thereof. As the pressurized ink stream I2 exits from the opening 14of the nozzle 1]. it is subjected to a modulated light beam 15, which issupplied from a high intensity light source I6. The high intensity lightsource 16 can be a gas laser such as a helium-neon gas laser. forexample. The laser is continuous since it cannot be turned on and off atthe desired frequency such as 100 kiloHertz. for example.

To intermittently supply the light beam I5 from the high intensity lightsource 16 to the stream 12. a modulator I7 is disposed between the highintensity light source 16 and the ink stream 12. The modulator 17 can beany suitable type in which a control 18 can effectively start and stopthe application of the light beam 15 to the ink stream I2.

The modulator 17 could include. for example. a glass with a chrome maskhaving a slit through which light could pass. The modulator 17 couldinclude an accoustic deflector. for example. which would shift the lightin response to the control I8 so that a focusing lens. for example.within the modulator I7 would not focus the beam at the slit in thechrome mask. When the accoustic deflector is inactivated by the control18. the beam is focused at the slit so that the beam passes through theslit and another lens front which it exits from the modulator 17.

Each time that the light beam 15 is applied to a segment ofthe inkstream I2, which must not be transparent to the light beam [5. the lightis converted to heat within the segment of the ink stream I2. Thisconversion of the light to heat increases the temperature of the segmentof the ink stream I2 to reduce its surface tension since the surfacetension of the ink stream I2 is inversely proportional to thetemperature of the ink stream 12.

Each time that the light beam I5 is applied to a segment of the inkstream 12 for a predetermined period of time. the surface tension of thestream I2 is reduced in the segment of the ink stream 12 subjected tothe light beam 15. The velocity of the ink stream 12 determines thesegment subjected to the light beam IS during the predetermined periodoftime. As a result of the reduction of the surface tension of thespaced segments of the ink stream 12. the ink stream 12 breaks intodroplets 19 of substantially uniform size with the droplets 19 havingsubstantially uniform spacing therebetween.

Prior to the droplets 19 being formed. the stream I2 enters a deflectionmeans 20, which is connected to a deflection signal source 21. in whichthe droplets 19 are charged at the moment of breakoff in the manner moreparticularly shown and described in the aforesaid Sweet patent. Thedeflection signal source 2] applies a signal to the deflection means 20to determine whether each of the droplets [9 falls into a gutter 22 fromwhich the droplet can be returned to the reservoir of the ink supply 10or strikes a recording surface such as a moving paper 23. which would bemoving horizon tally in a plane perpendicular to the drawing. Theposition on the paper 23 at which each of the droplets I9 strikes thepaper 23 also can be determined by the strength of the signal to thedeflection means 20 from the deflection signal source 21. Of course. ifa plurality of the ink streams 12 were used. then the deflection means20 would either cause the droplet 19 to fall into the gutter 22 or tostrike the same position on the paper 23 each time. In this case. thepaper 23 would be mov ing vertically with the plurality of nozzlesdisposed in a horizontal plane.

It should be understood that the control 18 for the modulator I7 and thedeflection signal source 21 must be synchronized so that the deflectionsignal source 21 provides the desired charge to the droplet 19 when itis within the deflection means 20. This desired charge results in thedesired deflection of the droplet 19.

Referring to FIG. 2. there is shown another embodiment in which the inkstream 12 is subjected to a light beam 30 from a high intensity lightsource 3I such as a light emitting diode or an injection laser. which isa solid state laser. for example. The light beam from the high intensitylight source 31 is modulated by a control 32. which is connected to thehigh intensity light source 31. Thus. the control 32 controls the periodof time that the high intensity light source 31 applies the light beam30 to a segment of the ink stream 12. which cannot be transparent to thelight beam 30.

Accordingly. the high intensity light source 31 is a source of energyfor creating a thermal disturbance or change in segments of the inkstream [2. This conversion of the energy of the light beam 30 to heat inthe ink stream 12 produces the desired increase in temperature in spacedsegments of the ink stream 12 to decrease the surface tension of thespaced segments of the stream 12 so that synchronous formation of thedroplets 19 again occurs. The remainder of the operation is the same asdescribed when the high intensity light source 16 of FIG. 1 is employedincluding synchronization ofthe control 32 with the deflection signalsource 2|.

Because the high intensity light source 31 can be relatively small anddoes not require a modulator between it and the ink stream 12 but usesonly the control 32 to regulate it. the high intensity light source 3]can be placed fairly close to the ink stream 12 such as about onediameter of the ink stream 12 from the ink stream 12. As a result. thereis no need for any lens with the high intensity light source 31 as isnecessary with the high intensity light source 16.

Referring to H08. 3 and 4. there is shown another form of the inventionin which an electric heater 35 such as a thin film resistive heater oran inductive heater. is employed. The heater 35 is disposed within anopening or passage 36 of a nozzle 37. which is connected to the inksupply 10 in the same manner as the nozzle H. and adjacent the exit ofthe opening 36.

Electrical energy is supplied to the heater 35 through contacts 38 and39. which are connected to a control 40. The control regulates when theheater 35 is turned on and off to control for how long the heat isapplied to the segment of the ink stream 12.

When the heater 35 is a thin film resistive heater. the heat transfer tothe ink stream 12 via conduction re sults in the surface of the segmentof the ink stream 12 becoming hotter than the interior of the ink stream12 in the segment to which heat is applied. Since the heat is applied tothe surface of the ink stream 12. this aids in reducing the surfacetension of the segment of the ink stream 12.

When the heater 35 is formed of a thin film of resistive material. it isformed of any suitable material. For example. it could be formed ofcopper or Nichrome.

When the heater 35 is an inductive heater. the heat transfer to the inkstream 12 is by induction. As a result. the thermal disturbance orchange in the segment of the ink stream [2 to which the inductive heatis applied is produced by conversion in the same manner as when light isapplied.

The nozzle 37 is preferably formed of an electrically insulatingmaterial such as quartz. for example. If the nozzle 37 is formed ofmetal. then the heater 35 must be electrically insulated from the nozzle37. In such an arrangement. a layer of insulating material such assilicon dioxide, for example. would be disposed around the portions ofthe heater 35 and the contacts 38 and 39 in engagement with the nozzle37.

The ink stream 12 can be transparent when used with this modification.The remainder of the o eration of this modification is the same as thatdescribed with re spect to FIG. I including the synchronization of thecontrol 40 with the deflection signal source 2].

Referring to FIGS. 5 and 6. there is shown a modification of thestructure of FIGS. 3 and 4 in that a heater 45. which can be a thin filmresistive heater or an inductive heater. does not completely surround anopening or passage 46 of a nozzle 47 but only partially surrounds it.Furthermore. the heater 45 is disposed within the nozzle 47 to form astreamline with the surface of the opening 46 as shown in FIG. 5.

The heater 45 is connected through contacts 48 and 49 to a control 50.The nozzle 47 is preferably formed of an electrically insulatingmaterial such as quartz. for example. If the nozzle 47 is formed ofmetal. then a layer of insulating material such as silicon dioxide. forexample. must be used to electrically insulate the heater 45 and thecontacts 48 and 49 from the nozzle 47.

The control 50 functions in the same manner as the control 40 todetermine when the heater 45 is on and off. The control 50 issynchronized with the deflection signal source 21. The remainder of theoperation of this modification is the same as described with respect toFIG. 1.

The heater 45 requires a smaller quantity of power than that requiredwhen the heater 35 is employed. By heating a portion of the surface ofthe segment of the ink stream [2 when the heater 45 is a thin filmresisti\ e heater. the temperature of the portion of the segment of theink stream [2 to which the heat is applied is increased sufficiently todecrease the surface tension of the segment of the ink stream 12.

While the heater 45. which only partially surrounds the stream 12. hasbeen shown as being disposed to form a streamline with the surface ofthe opening 46. it should be understood that the heater 45 could bedisposed within the opening 46 in the nozzle 47 as the heater 35 ofFIGS. 3 and 4. Similarly. while the heater 35 has been shown as beingdisposed within the opening 36. it should be understood that the heater35 could be disposed to form a streamline with the opening 36 as theheater 45 of FIGS. 5 and 6.

Instead of disposing the heater 45 within the nozzle 47 to form astreamline with the surface of the opening 46. it should be understoodthat the high intensity light source 31 could be disposed within thenozzle 47 rather than the heater 45. Thus. it is not necessary for thehigh intensity light source 3] to be exterior of the nozzle as shown inFIG. 2.

To show how the reduction in surface tension of a liquid stream can formdroplets with substantially uniform spacing and of substantially uniformsize. a stream of water to which a thin film resistive heater of 809?nickel and 20% chrome (Nichrome) applies heat will be considered as anexample. When the temperature of water is heated from 20 C. to 30 C. itssurface tension decreases from 72.75 dynes/cm. to 7|.l8 dynes/cm. for adecrease of 2.4%. Since the internal pressure of a jet stream of liquidis directly proportional to the surface tension of the stream andinversely proportional to the diameter of the stream. a temperature riseof l0 C. at the surface of the water stream has the same effect oninternal pressure as an increase in the diameter of the stream of 2.47:.

11' the stream is assumed to have a diameter of 1 mil. a 2.45; variationin the surface tension of a 1 mil section or segment at 4 mil intervalsproduces a diameter increase of 2.40 within approximately 5 microsecondssince it is known. as explained hereinafter. that the time constant ofthe stream instability causes a disturbance to double in magnitude inabout 5 microseconds and since the initial 2.4; reduction in surfacetension is equivalent to an initial diameter increase of 2.4

As explained in Breakup of a Laminal Capillary Jet of a ViscoelasticFluid" by M. Goldin et al. on pages 689-71 1 of \'ol. 38. Part -1 of theJournal of Fluid Mechanies l 1969 I. the growth rate of a diameterdisturbance is given by (ll 1) 11,,1 where d is the size of thedisturbance when the time 1 and 1(1) is the size of the disturbance whentime is equal to or greater than 0. Ignoring viscosity. the coefficienta.,* is given in equation 1 19) on page 693 ofthe aforesaid article bywhere 0' is the surface tension and is approximately 70 dyne cm forwater. p is the density and is l gm cm for water. and a is the radius ofthe stream. For a jet diameter of 1 mil. a 0.5 mil 12.5 X 10 cm. Thus.

a,,* 170/: (12.5 X lu 0.134 x 10'.

A disturbance of size 11,, doubles when a 2. Accordingly. a,,*! ln 2 sothat I 1n 2/a,,*. With a 0.134 X 10" r 5.17 X 10" sec. Thus. for a 1 mildiameter jet of water. a diameter disturbance doubles in size in about 5X sec.

Since the random breakup of a jet stream until it has droplet formationis about 100 microseconds after leaving a nozzle. it is necessary thatthe breakup of the stream into the controlled droplet formation occurbefore this time. Therefore. if the disturbance in the diameter of thestream were to be reduced front the point of droplet formation back toits exit from the nozzle by one-half every 5 microseconds since this isthe opposite of doubling the disturbance every 5 microseconds from thenozzle to the point ofdroplet formation. the reduction of the diameterof the stream adjacent the outlet of the nozzle to obtain controlleddroplet formation in 100 microseconds can be ascertained. Thus. /a 10stream diameters so that a perturbation of 10 mil in a stream having adiameter of 1 mil is adequate to produce droplets of uniform spacing in100 microseconds from this perturbation. Accordingly. a 3 X 10" milperturbation would completely dominate random disturbances and causedroplet formation in about 75 microseconds.

To ascertain ifthere will be sufficient heat flow in the water withinthis period of time. the 1 mil diameter nozzle. which supplies the jetstream of 1 mil diameter. is approximated by two plates being l milapart and utilizing the temperature distribution in a plate as shown inFIG. 104 on page of Mathematical and Physical Principles of EngineeringAnalysis by Walter C. Johnson (First Edition. fourth impression). FIG.104 shows the temperature distribution in a plate as a function of timestarting with a uniform temperature in the plate and maintaining thesurfaces of the plate thereafter at a constant temperature. which isdifferent from the ini tial uniform temperature. There is a plurality ofcurves shown with each being for a different value of Br in which B is aconstant and l is the time with B 1r k/L rp. In this equation. k is thethermal conductivity and is (1.04 X10 watt cm. K" at 20 C. for water. Lis the spacing between the plates so it is l mil. c represents thespecific heat and is 4.18 watt sec gm T. for water. and p is the densityand is equal to 1 gm emf for water. Thus. for water at 20 C. B equals2.3 X 10 sec". When B! 10 the temperature from FIG. 10-1 of MathematicalPhysical Principles of Engineering Analysis may be approximated by astraight line connecting the surface temperature 6 at 0 and .v/L 0.1.Thus. with B: 10*. r =4.35 X l0 see.

If the surface temperature of the water is raised C. above the interior.the heat energy put into a 1 mil length of a water stream. which has adiameter of l mil with the heat being applied to only the outer layer ofthe stream with a thickness of0.l mil. is the product of [20C X 0.5).(2.5 X10 cm). (1r X 2.5 X l0 cm). (2.5 X 10" cm). and (4.18 watt sec gnfC"') with (20C X 0.5) being the average temperature in the 0.1 mil layerof water. (2.5 X 10* cm) being the thickness of the outer layer of thewater to which the heat is applied. (1r X 2.5 X 10 cm) being the widthof the stream of water and is its circumference. (2.5 X 10 cm) being the1 mil length of the stream of water to which the heat is applied. and(4.18 watt sec gm" C '1 being the specific heat of water. This producesa heat energy of 2.05 X 10' watt sec. With this heat energy of 2.05 X10' watt sec being applied for 4.35 X 10" seconds. the power input tothe water during this time period is approximately 0.05 watt. with a505i duty cycle and 50% efficiency in transferring heat from a thin filmresistive heater to the water. a power input of 0.05 watt to the streamseems adequate.

Thus. sufficient heat energy can be supplied to the water stream toreduce its surface tension 2.4% within the necessary time. If a thinfilm of Nichrome (8091 nickel. 20V: chrome) with a thickness of l micronis used as the heater. of the heat therein is removed when B: equals 1.0according to FIG. 104 of the Mathematical and Physical Principles ofEngineering Analysis by Johnson. For Nichromc. B is approximately 0.3 X10'" since k is equal to 0.12 watt cm. K. L is equal to 10* cm. (1micron). and up is equal to approximately 4 watt sec emf. With B X 0.3 X10 .115 33 X l0 sec. so that 70% 0f the heat would be removed from a 1micron thick heater of Nichrome in 33 nanoseconds.

The resistance of the thin film resistive heater is obtained from theequation of R i'l/A. In the equation. r is the resistivity of Nichromeand is X 10 ohm/meter. l is the length of the heater. and A is the areaof the resistive heating element of the heater.

The resistive heater 35 in FIG. 4 can be considered as two resistiveheaters of length rr 11/2 electrically in parallel where (I is thediameter of the nozzle. The nozzle diameter is 1 mil or 25 microns sinceit surrounds the stream of 1 mil diameter. Thus. each heater is about37.5 microns long.

Since the heater should extend for the length of the segment of thestream to which the heat is applied. it would be 1 mil or 25 microns.and this is considered to be its width insofar as determining itsresistance. The thickness of the heater is 1 micron since this is thethickness utilized in calculating B in Br for Nichrome. Thus. from R NH.the resistance R of each halfof the thin film resistive heater would be1.5 ohms.

Accordingly. the thin film resistive heater is capable of supplyingsufficient heat in 33 nanoseconds to reduce the surface tension of thewater stream 2.4" Since this is a very small portion of time incomparison with the available period of 5 microseconds to reduce thesurface tension of the water stream 2.4%. the thin film resistive heatercan be utilized effectively through modulating its supply of heat.

Additionally. with the resistance of 0,75 ohm of the heater 35 of FIG.4. an application of \oltage of about 0.2 volt could he used and requireonly 267 milliamps to produce the necessary heat energy of 0.05 watt.Thus. no large current or voltage is required.

It should be understood that the calculations of the foregoing exampleare an approximation and could he an order of magnitude different. Thus.the heat required could be an order of magnitude different from thatcalculated but this still would not require a large current or voltage.

While the foregoing has discussed the stream as being formed of water.it should he understood that an ink stream could have a differentsurface tension. However. the same type of calculations would be made toobtain the necessary size of a thin film resistive heater such as theheater 35. for example.

While the period oftime for applying a source of heat or light to thestream can be for the same period oftime as it is not applied. it shouldbe understood that such is not a requisite for satisfactory operation.Thus. the period oftime during which the energy is applied could heshorter or longer than the period of time during which the energy is notapplied to the stream. it is only necessary that the formation of thedroplets occur substantially before random break up of the dropletswould occur and that the disturbance produced be greater than anynatural disturbance of the stream.

The diameter of each formed droplet is a function of both the velocityand the diameter of the stream and the frequency with which the energyis applied to the stream. However. the quantity ofenergy does not affectthe diameter of the droplet.

The quantity of the energy only determines the hreakoff point of thedroplet from the stream. That is. as the quantity of the energy isincreased. the hreakoff point of the droplet occurs closer to theopening of the nozzle. Of course. the quantity of energy must be sufficient to produce synchronization of droplet formation.

It should be understood that the wave form of the modulated powerapplied to the stream can be of varying shapes such as a square wavepulse or a sine wave. for example. The specific shape of the powerapplied to the stream influences the formation of the droplet. and aparticular power wave form can be chosen to minimize satellites duringdroplet formation.

An advantage of this invention is that substantially uniform spacing andbreakoff of the droplets of an ink stream is obtained so that eachdroplet can he controlled as to whether it is applied to a recordingsurface and the area to which it is applied. Another advantage of thisinvention is that it has no effect on the jet stream of any adjacentnozzle whereby a plurality of ink streams can be disposed close to eachother.

While the invention has been particularly shown and described withreference to preferred embodiments thereof. it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

l. A method of forming droplets at a substantially constant breakoffpoint and with substantially uniform distances from each other from aliquid stream exiting from an opening or the like including:

selectively altering the surface tension of spaced segments of thestream by initially reducing the surface tension of each of the spacedsegments of the stream as it passes a predetermined portion of its pathand before random break up of the stream into droplets would occur afterthe stream leatcs the opening;

and controlling the amount of initial reduction of the surface tensionof each of the spaced segments of the stream to control the breakot'fpoint of the droplets from the stream at a substantially constantbreakoff point and with substantially uniform distances from each other.

2. The method according to claim 1 including:

applying a source of energy directly to each of the spaced segments ofthe liquid stream to produce a thermal change in the spaced segments ofthe stream before random break up of the stream into droplets wouldoccur after the stream leaves the opening:

periodically applying the source of energy to each of the spacedsegments of the stream as it passes the predetermined portion of itspath for a constant period of time to initially reduce the surfacetension of the stream in the spaced segments of the stream to formdroplets at substantially uniform distances from each other;

and controlling the quantity of energy applied from the source of energyduring each constant period of time to control the breakoff point.

3. The method according to claim 2 including applying the sourceofenergy to the spaced segments of the stream after the stream leavesthe opening.

4. The method according to claim 3 including applying light from a highintensity light source as the source of energy to produce the thermalchange in the stream.

5. The method according to claim 2 including controlling the quantity ofenergy applied during the constant period of time and controlling thetime period of the constant period of time to obtain the desiredsynchronization and breakoff point of the droplets from the stream.

6. The method according to claim 2 including applying the source ofenergy to the spaced segments of the stream just before the stream exitsfrom the opening.

7. The method according to claim 6 including applying the source ofenergy to the spaced segments of the stream around the entire peripheryof the stream.

8. The method according to claim 7 including applying heat to the spacedsegments of the stream as the source of energy.

9. The method according to claim 6 including applying the source ofenergy to the spaced segments of the stream around only a portion of theperiphery of the stream.

10. The method according to claim 9 including applying heat to thespaced segments of the stream as the source of energy.

ll. An apparatus for forming droplets at a substantially constantbreakoff point and with substantially uniform distances from each otherfrom a liquid stream including:

means to supply the liquid stream through an opening or the like;

and means to selectively alter the surface tension of spaced segments olthe stream to form droplets at substantially uniform distances from eachother and of substantially uniform size. said selecti\el altering meansbeing applied to each of the spaced segments of the stream as it passesa predetermined portion of its path to initially reduce the surfacetension of each of the spaced segments l el'ore random break up of thestream into droplets would occur after the stream exits from theopening.

l2. The apparatus according to claim ll in which said selectivelyaltering means includes means to periodically apply a source of energydirectly to each ol'the spaced segments ol the liquid stream for aconstant period of time at the predetermined portion of its path toproduce a thermal change in the stream to reduce the surface tension ofeach of the spaced segments of the stream to which the energy isapplied.

IS. The apparatus according to claim T2 in which said applying meanscomprises energy supply means disposed exterior of the opening to applyenergy to the spaced segments ofthe stream before random break up of theliquid stream into droplets would occur.

14. The apparatus according to claim U in which:

. 12 said energy supply means includes:

a high intensity light source: and means to modulate said high intensitylight source to apply the light of said high intensity light sourceperiodically for the constant period of time.

[5. The apparatus according to claim 14 in which said high intensitylight source is a continuous light source.

[6. The apparatus according to claim [2 in which said applying meansincludes heating means disposed within the opening [7. The apparatusaccording to claim 16 in which said heating means is disposed adjacentthe exit of the opening.

18. The apparatus according to claim 16 in which said heating meanscompletely surrounds the liquid stream to apply heat to the entireperiphery of each of the spaced segments of the liquid stream.

19. The apparatus according to claim [6 in which said heating means onlypartially surrounds the liquid stream to apply heat to only thesurrounded portion of the periphery of each of the spaced segments ofthe liquid stream.

1. A method of forming droplets at a substantially constant breakoffpoint and with substantially uniform distances from each other from aliquid stream exiting from an opening or the like including: selectivelyaltering the surface tension of spaced segments of the stream byinitially reducing the surface tension of each of the spaced segments ofthe stream as it passes a predetermined portion of its path and beforerandom break up of the stream into droplets would occur after the streamleaves the opening; and controlling the amount of initial reduction ofthe surface tension of each of the spaced segments of the stream tocontrol the breakoff point of the droplets from the stream at asubstantially constant breakoff point and with substantially uniformdistances from each other.
 2. The method according to claim 1 including:applying a source of energy directly to each of the spaced segments ofthe liquid stream to produce a thermal change in the spaced segments ofthe stream before random break up of the stream into droplets wouldoccur after the stream leaves the opening; periodically applying thesource of energy to each of the spaced segments of the stream as itpasses the predetermined portion of its path for a constant period oftime to initially reduce the surface tension of the stream in the spacedsegments of the stream to form droplets at substantially uniformdistances from each other; and controlling the quantity of energyapplied from the source of energy during each constant period of time tocontrol the breakoff point.
 3. The method according to claim 2 includingapplying the source of energy to the spaced segments of the stream afterthe stream leaves the opening.
 4. The method According to claim 3including applying light from a high intensity light source as thesource of energy to produce the thermal change in the stream.
 5. Themethod according to claim 2 including controlling the quantity of energyapplied during the constant period of time and controlling the timeperiod of the constant period of time to obtain the desiredsynchronization and breakoff point of the droplets from the stream. 6.The method according to claim 2 including applying the source of energyto the spaced segments of the stream just before the stream exits fromthe opening.
 7. The method according to claim 6 including applying thesource of energy to the spaced segments of the stream around the entireperiphery of the stream.
 8. The method according to claim 7 includingapplying heat to the spaced segments of the stream as the source ofenergy.
 9. The method according to claim 6 including applying the sourceof energy to the spaced segments of the stream around only a portion ofthe periphery of the stream.
 10. The method according to claim 9including applying heat to the spaced segments of the stream as thesource of energy.
 11. An apparatus for forming droplets at asubstantially constant breakoff point and with substantially uniformdistances from each other from a liquid stream including: means tosupply the liquid stream through an opening or the like; and means toselectively alter the surface tension of spaced segments of the streamto form droplets at substantially uniform distances from each other andof substantially uniform size, said selectively altering means beingapplied to each of the spaced segments of the stream as it passes apredetermined portion of its path to initially reduce the surfacetension of each of the spaced segments before random break up of thestream into droplets would occur after the stream exits from theopening.
 12. The apparatus according to claim 11 in which saidselectively altering means includes means to periodically apply a sourceof energy directly to each of the spaced segments of the liquid streamfor a constant period of time at the predetermined portion of its pathto produce a thermal change in the stream to reduce the surface tensionof each of the spaced segments of the stream to which the energy isapplied.
 13. The apparatus according to claim 12 in which said applyingmeans comprises energy supply means disposed exterior of the opening toapply energy to the spaced segments of the stream before random break upof the liquid stream into droplets would occur.
 14. The apparatusaccording to claim 13 in which: said energy supply means includes: ahigh intensity light source; and means to modulate said high intensitylight source to apply the light of said high intensity light sourceperiodically for the constant period of time.
 15. The apparatusaccording to claim 14 in which said high intensity light source is acontinuous light source.
 16. The apparatus according to claim 12 inwhich said applying means includes heating means disposed within theopening.
 17. The apparatus according to claim 16 in which said heatingmeans is disposed adjacent the exit of the opening.
 18. The apparatusaccording to claim 16 in which said heating means completely surroundsthe liquid stream to apply heat to the entire periphery of each of thespaced segments of the liquid stream.
 19. The apparatus according toclaim 16 in which said heating means only partially surrounds the liquidstream to apply heat to only the surrounded portion of the periphery ofeach of the spaced segments of the liquid stream.